OFDM offers the advantages of improved downlink system capacity, coverage and data rates for packet data services with high spectral efficiency. This is due at least in part to a substantially rectangular spectrum occupancy and the ability to achieve a low-cost implementation using the Fast Fourier Transform (FFT). OFDM has been exploited for wideband data communications over mobile radio channels, high bit rate digital subscriber lines (HDSLs), asymmetric digital subscriber lines (ADSLs), and digital broadcasting. Reference in this regard can be made to the following publications: J. A. C. Bingham, “Multicarrier modulation for data transmission: An idea whose time has come,” IEEE Commun. Mag., vol. 28, pp. 5-14, May 1990; J. S. Chow, J. C. Tu, and J. M. Cioffi, “A discrete multitone transceiver system for HDSL applications,” IEEE J. Select. Areas Commun., vol. 9, pp. 895-908, August 1991; W. Y. Chen and D. L. Waring, “Applicability of ADSL to support video dial tone in the copper loop,” IEEE Commun. Mag., vol. 32, pp. 102-109, May 1994; and to W. Y. Zou and Y. Wu, “COFDM: An overview,” IEEE Trans. Broadcast., vol. 41, pp. 1-8, March 1995.
OFDM partitions the entire bandwidth into parallel independent subcarriers to transmit parallel data streams. The relatively longer symbol duration provides increased immunity to inter-symbol interference (ISD. OFDM has received considerable attention as an air interface for evolution of the Universal Mobile Telecommunications System (UMTS) mobile radio systems in the Third Generation Partnership Project (3GPP) forum. Reference in this regard can be made to 3GPP TR 25.892 v0.1.1 (2003-02), “Technical Specification Group Radio Access Network: Feasibility Study for OFDM for UTRAN Enhancement (Release 6)”.
Channel estimation plays an important role in OFDM systems that is essential to bit and power allocation and signal detection. Without perfect knowledge of channel state information (CSI), an OFDM system may be inoperative or may incur significant performance loss. Pilot-based channel estimations are widely used to estimate the channel and equalize the received signal for OFDM systems, where the pilot signals are uniformly distributed in the time and/or frequency domain (FD) based on coherent time and frequency. Reference in this regard can be made to J. J. Beek, O. Edfors, M. Sandell, S. K. Wilson, and P. O. Borjesson, “On Channel Estimation in OFDM Systems,” IEEE VTC95-Fall, pp. 815-819, Chicago, USA, July 1995, and to O. Edfors, M. Sandell, J. J. Beek, S. K. Wilson, and P. O. Borjesson, “OFDM Channel Estimation by Singular Value Decomposition,” IEEE VTC96-Spring, pp. 923-927, Atlanta, USA, April 1996.
A lowpass filter in a transform domain was proposed for suppressing inter-carrier interference (ICI) and additive white Gaussian noise (AWGN) by O. Y. Zhao, and A. Huang, “A Novel Channel Estimation Method for OFDM Mobile Communication Systems Based on Pilot Signals and Transform-Domain Processing,” IEEE VTC97-Spring, pp. 2089-2093, Phoenix, USA, May 1997. Second-order interpolation has been shown, see M. Hsieh, and C. Wei, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems,” IEEE Transactions on Broadcasting, Vol. 48, No. 3, September 2002, to outperform the linear interpolation with piecewise constant approximation, as in J. Rinne, and M. Renfors, “Pilot Spacing in Orthogonal Frequency Division Multiplexing Systems on Practical Channels,” IEEE Transactions on Consumer Electronics, Vol. 42, No. 4, November 1996. Time-domain interpolation is obtained by passing the FD channel estimates into the time domain (TD) through IFFT, zero padding, and converting back to the frequency domain through FFT (see Sinem Coleri, M. Ergen, A. Puri, and A. Bahai, “Channel Estimation Techniques based on Pilot Arrangement in OFDM Systems,” IEEE Transactions on Broadcasting, Vol. 48, No. 3, September 2002). The performance of the time-domain interpolation has been evaluated by R. Steele, “Mobile Radio Communications,” Pentech Press Limited. London, England, 1992.